The invention relates to a method of forming an impermeable coating on the wall of a borehole penetrating subsurface earth formations.
During the course of well drilling operations, the wall of the borehole being drilled is generally sealed and stabilized by means of a protective steel casing which is lowered through the borehole and cemented in place after retrieval of the drilling assembly. Setting a steel casing in a well is a time consuming and expensive procedure and numerous attempts have been made to eliminate the need for such well casings. U.S. Pat. No. 3,774,683 discloses a method of stabilizing a borehole wall by means of a lining of cement reinforced with fibers. In accordance with this known stabilization process, a hydraulic cement plug is placed in the borehole and a core is drilled in the plug after the cement hardens. U.S. Pat. No. 3,302,715 discloses a method of solidification of a mud cake alongside a borehole wall by fusing sulphur particles contained therein. U.S. Pat. No. 3,126,959 discloses a method of forming a continuous plastic casing in a borehole by extruding plastic material alongside the borehole wall.
Although these known borehole stabilization systems provide useful alternatives to conventional steel casings, they still have the inherent disadvantage of application of equipment which is inserted in the well after retrieving the drilling assembly therefrom. However, pulling a drill string from a borehole is a time consuming and hazardous procedure. A major hazard resides in the fact that the upwardly moving drill string may create a considerable underpressure at the bottom of the hole. If the pressure inside the hole becomes lower than the formation pressure, ingress of formation fluids into the well may easily cause damage to the borehole wall and may occasionally lead to a well blowout.